This invention relates to an embossing process for imparting a textured surface to heat shapable thermoplastic sheet.
Systems are well established for passing thermally softened thermoplastic sheet through a nip formed by a rotating metal embossing roll having an engraved surface pattern and an opposing rubber-faced backup roll to press the sheet by rubber compression against the embossing roll and imprint the sheet with the pattern. At commercial line speeds the residence time of the sheet against the engraved surface in these systems is relatively short.
Polyvinyl butyral (PVB) sheet for laminated safety glass is representative of sheet provided with a textured surface in processes of the type described. Note, for example, U.S. Pat. No. 5,455,103 to Hoagland et al for disclosure of such a system in the Examples. The rough, textured surface provides channels between glass layers and the PVB sheet for air removal during formation of the layered safety glass which then is conventionally removed by melting the surface smooth during elevated temperature laminating. At commercially economical sheet line processing speeds, it is difficult in the embossing nip to impart a rough surface to such sheet which is sufficiently deep and permanent to accommodate deairing during glass laminating. Moreover, significant compression of the rubber of the backup roll operating at elevated temperature encountered in the embossing nip causes relatively rapid deterioration of the rubber after short operating intervals.
It would be desirable to provide an embossing process for heat shapable thermoplastic sheet which avoids or minimizes problems encountered in prior art embossing systems.